Question: If $a + 7b = 4$ and $7x + 4y = 9$, what is $-21x + 56b - 12y + 8a$ ?
Explanation: $= 8a + 56b - 21x - 12y$ $= (8) \cdot (a + 7b) + (-3) \cdot (7x + 4y)$ $= (8) \cdot (4) + (-3) \cdot (9)$ $= 32 - 27$ $= 5$